A high separation factor and a high permeation rate to gases are required for a gas separation membrane.
For satisfying such a performance, a structure may be considered in which a membrane layer substantially contributing to the separating function is provided at a thickness as thin as possible and supported on a porous layer.
Various methods have been devised for the process of producing membranes of such a structure.
There can be mentioned, for instance, a method of overlaying thin films on a porous layer which are prepared separately to each other, a method of forming an anisotropic membrane at once in which a surface layer and a porous layer are present together and a method of forming a thin film by carrying out in situ polymerization, on a porous layer, from a monomer by way of various processes or a method of forming a thin film by coating a solution of polymer and then evaporating a solvent.
Silicon-containing polymers are generally known as the material having the highest gas permeability coefficient (value represented by the volume of a gas in cm.sup.3 which permeates through a film of 1 cm thickness, at the film area 1 cm.sup.2 for one second with the pressure difference of 1 cmHg), and polysiloxane, block copolymers of polysiloxane with polycarbonate, styrene, phenolic resin or the like, and polyvinyltrimethylsilane have, for example, been employed as the material for the gas separation membrane.
Among these silicon-polymers, although polysiloxane is the material having the highest permeability coefficient, its possible thickness when formed into a thin film is restricted due to the low strength thereof. Materials improved with the defect in a strength include, for example, polysiloxane-polycarbonate copolymer, polysiloxane-styrene copolymer and polysiloxane-phenolic resin copolymer and they are formed into a thin film and overlayed with a porous film prepared separately for use. Further, polyvinyltrimethylsilane can be formed into an anisotropic membrane of a thin film thickness and a dense structure and used, as the gas separation membrane.
The membrane material of a polymer having the main repeating unit of the formula ##STR3## and/or formula ##STR4## is a polymer having carbon-carbon bonds in the main chain and silicon atoms on the side chain of the polymer in the same manner as in polyvinyltrimethylsilane having the repeating unit of the formula ##STR5## and the similar separating function has been anticipated. As the result of the study, we have however found that a characteristically excellent separating function can be obtained as compared with polyvinyltrimethylsilane and have accomplished the present invention.